Torsion oscillator stabilization

ABSTRACT

A torsion oscillator (FIG.  1 ) is stabilized in operation by determining the current resonant frequency ( 62 ); in a first procedure, observing the oscillator for change in resonant frequency ( 64 ), and then restoring the amplitude and median offset ( 66 ) without changing the drive frequency. In an alternative procedure, after determining the resonant frequency ( 62 ); setting the drive frequency close to but offset from the current resonant frequency ( 74 ), observing the oscillator for change in resonant frequency ( 76 ), and the restoring the close offset to the changed resonant frequency ( 78 ). By operating slightly off peak, the direction of resonant change is immediately known. The first procedure has less difficulties in implementation, but requires more power.

FIELD OF THE INVENTION

[0001] Torsion oscillators are typically driven by electrical signalsapplied at the resonant frequency of a body mounted between torsionmembers. This invention addresses the stabilization of torsionoscillators as their resonant frequency varies.

BACKGROUND OF THE INVENTION

[0002] Torsion oscillators are known, although not widely employed. U.S.Pat. Nos. 4,762,994 to Byerly et al., 5,543,956 to Nakagawa et al. and5,767,666 to Asada et al. are illustrative. An illustration of agalvanometric torsion oscillator is shown in FIG. 1. (The termgalvanometric is believed to be a reference to coils on the turningmember operated in the manner of a common galvanometer.)

[0003] The torsion oscillator of FIG. 1 comprises a central rectangularplate 1 suspended by two extensions 3 a, 3 b of the material of plate 1.Extensions, 3 a, 3 b are integral with a surrounding frame 5. Typically,the plate 1, extensions 3 a, 3 b and frame 5 are cut or etched from asingle silicon wafer. A coil 7 of conductive wire and a region 9 ofreflective mirror material are placed on the central plate.

[0004] This entire assembly is located inside a uniform magnetic field11 (shown illustratively by lines with arrows), such as from opposingpermanent magnets (not shown). When a current passes through coil 7, aforce is exerted on coil 7 which is translated to plate 1 since coil 7is attached to plate 1. This force causes rotation of plate 1 aroundextensions 3 a, 3 b which twist with reverse inherent torsion.

[0005] Other means may be employed to make such a system oscillate, suchas static electricity or external magnetic fields. Various ones of suchmeans are known in the prior art. The use of a coil drive by electriccurrent in the embodiments disclosed herein should be consideredillustrative and not limiting.

[0006] The spring rate of extensions 3 a, 3 b and the mass of plate 1constitute a rotational spring-mass system with a specific resonantfrequency. Plate 1 can be excited to oscillate at the resonant frequencywith an alternating level passing through the coil and having afrequency at the resonate frequency or having some other frequency, suchas harmonic at the resonate frequency. Where the input frequency variesfrom the resonant frequency and is substantial in power, plate 1oscillates at the input frequency but drive level to coil 7 must behigher to achieve the same sweep (extent of oscillation) of plate 1. Thedevice functions as a laser scanner when a laser is directed at theoscillating surface of mirror 9, thereby replacing the much bulkierrotating polygonal mirror widely used in laser printers and copiers.Torsion oscillators also have other applications, such as to drive aclocking device, in which mirror 9 would not be used.

[0007] The angle of mirror 9 moves sinusoidally with respect to time ata certain amount of sweep (termed amplitude), in a certain repetitionrate (termed frequency), and with a potential lack of symmetry withrespect to the using apparatus (termed median offset). These elementsmust be stabilized for useful operation. But the characteristics of atorsion oscillator can vary significantly from manufacturing tolerancesand changing environmental conditions. Moreover, the direction offrequency drift is not readily determined since amplitude falls fordrift to both higher and lower frequency. This invention provides twoalternative control procedures which stabilize operation as the resonantfrequency shifts during use.

DISCLOSURE OF THE INVENTION

[0008] In accordance with a first control procedure of this invention,drift is observed by sensing a reduction in amplitude. In response theoriginal drive frequency is maintained and previous amplitude isrestored by an increase in drive level and any undesired median offsetis eliminated by an opposite change in the median of the drive level.This is the preferred control procedure where drift will not be so greatas to overcome available power or power-use limits of the oscillator.This procedure is not preferred where the necessary level of power isimpractical or the associated financial costs are too high.

[0009] In accordance with a second control procedure of this invention,the frequency of the drive signal to the torsion oscillator is set asmall amount offset below or above resonate frequency. The direction ofthis frequency offset is known. Operation of the oscillator is observedto determine the amplitude of the oscillator (this may be inferred fromthe time the light of a scan beam activates a sensor twice). When theoffset is below and the amplitude increases the drive frequency isreduced to stay below the new resonant frequency, when the offset isbelow and the amplitude decreases, the drive frequency is increased toremain close to the new resonant frequency. Similarly when an aboveoffset is used and the amplitude increases, the drive frequency isincreased to stay below the new resonant frequency. When an above offsetis used and the amplitude decreases, the drive frequency is reduced toremain close to the new resonate frequency.

[0010] Operation of the device using the oscillator is necessarily atthe power input frequency for both of the foregoing control procedures.Accordingly, operating frequency of the using device for the firstcontrol method remains fixed, while operating frequency for the secondcontrol method varies continually.

DETAILED DESCRIPTION OF THE DRAWINGS

[0011] The details of this invention will be described in connectionwith the accompanying drawings, in which FIG. 1 describes arepresentative torsion oscillator known in the prior art; FIG. 2illustrates a typical oscillator resonant frequency response withvarying temperature; FIG. 3 is a schematic illustration of a systemusing this invention; FIG. 4 illustrates scan angle versus time of therotation of a typical torsion oscillator; FIG. 5 illustrates a firstcontrol sequence in accordance with this invention; and FIG. 6illustrates a second control sequence in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0012] The natural resonant frequency of a torsional oscillator istypically very sharply defined, meaning that scan amplitude dropssignificantly if the drive lever is held constant but drive frequencyvaries to either side of the resonant frequency. Also, the naturalresonant frequency of a particular device can change easily withenvironmental conditions such as temperature. Typically, because ofthermal expansion of material in the oscillator, resonant frequencydrops with increasing temperature.

[0013]FIG. 2 is a plot of such a typical system response with drivefrequency as the horizontal axis and amplitude as the vertical axis, atconstant drive level. The left, dashed graph shows the response of thesystem at a temperature T1, which is the lowest temperature illustrated.The solid graph shows response of the system at a temperature T2 whichis higher than T1 but lower than T3, T2 being roughly centered intemperature between T1 and T3. The right, dashed graph shows theresponse of the system at the temperature T3.

[0014] As is apparent from FIG. 2, where the drive frequency is at theresonant frequency and the resonant frequency changes, at constant drivelevel the amplitude is substantially the same at different resonantfrequencies driven at the resonant frequency. A new resonant frequencycaused by change in temperature or other ambient factors could be eitherhigher or lower. Stabilization at the same drive level would be bychanging the drive frequency to the new resonant frequency, but thatfrequency could either be higher or lower.

[0015] In accordance with a first control procedure of this invention,the ambiguity of frequency drift is eliminated as a factor bymaintaining the drive frequency at the original frequency whilemaintaining the amplitude by increasing power level and, when needed,changing the median drive level to eliminate undesired median offset.This method allows the device using the torsional oscillator to operatemostly at a single frequency, with only periodic adjustments to thisoperating frequency.

[0016] In accordance with a second control procedure of this inventionthe torsion oscillator is driven at a frequency having a small offsetfrom the resonant frequency in a known sense of higher or lower. Thedirection of resonant frequency shift is then known and the drivefrequency changed accordingly, but with a small offset from the newresonant frequency so that the stabilization can be continued around thenew frequency. As is apparent from the rising and falling responsearound the resonant frequency, if the offset frequency is below resonantfrequency and amplitude increases, the resonant frequency has decreased;if the offset frequency is below resonant frequency and amplitudedecreases, the resonant frequency has increased. The drive frequenciesare moved to restore the small offset. When the offset frequency isabove the resonant frequency, the move of drive frequency to restore thesmall offset is in the opposite direction. The offset should be enoughto assure stabilization as described with an additional amount to besure tolerance variations will not affect operation. Operating at anoffset does reduce amplitude or increase power consumption, but notmaterially for many if not all applications if the offset is small.

[0017] This method is operative with a device using the torsionaloscillator which continuously accommodates the varying operatingfrequency of the oscillator.

Control System

[0018] Apparatus to control the torsion oscillator in accordance withthis invention would necessarily involve electronic control, such as amicroprocessor or combinational logic in the form of an ApplicationSpecific Integrated Circuit (commonly termed an ASIC). Details of suchimplementation may be conventional.

[0019] A representative, schematic illustration of such animplementation is shown in FIG. 3. An oscillator 20 may be that ofFIG. 1. A laser 22 trains on the reflective surface (mirror 9, FIG. 1).Scan ampliture of light reflected is shown by broken lines 24 a, 24 bindicating the outer limits of the reflected light and arrow 26indicating the largest angle of the scan. Middle line 27 is at a zeroangle of scan.

[0020] Two sensors, A and B, are located within the angle of scan.Sensor A upon receiving the reflected light creates an electrical signalon line 28 to control logic 30, which may be a microprocessor. Sensor B,upon receiving the reflected light, also creates an electrical signal online 32 to control logic 30.

[0021] Control logic 30 creates a signal defining required frequency online 34. Line 34 connects to frequency generator 36, which creates asignal of the defined frequency on line 38. The signal on line 38 isconnected to amplitude adjust system 40. Control logic 30 also creates asignal defining required amplitude on line 42. Line 42 connects toamplitude adjust system 40, which creates a signal of the definedfrequency and the defined amplitude on line 44. The signal on line 44 isconnected to offset adjust system 46. Control logic 30 also creates asignal on line 48 defining required offset. The signal on line 48 isconnected to offset adjust system 46.

[0022] Offset adjust system 46 creates a signal of the definedfrequency, the defined amplitude, and the defined offset on line 50.Line 50 is connected to power drive system 52, which creates an analogsignal corresponding to this information on line 54, which controlsoscillator 20. That may be a current or voltage signal, depending on thecharacteristics of oscillator 20. With respect to the oscillator of FIG.1, that would be a current signal of varying levels delivered to coil 7depending on the information defined by control logic 30.

[0023] For purposes of discussion, FIG. 4 illustrates time versus scanangle of mirror 9 of a representative system corresponding to FIG. 3having two light sensors corresponding to A and B of FIG. 3 located toreceive scanned laser light beams from mirror 9 near the extremes of thescan. The electronic control logic 30 measures the time interval ofsignals from the light sensors, as well as controlling the drive leveland frequency to coil 7.

[0024] As shown in FIG. 4, a time diagram of scan angle relative to thebeam encountering the sensor is defined. A first sensor, sensor A, isknown to be at a predetermined scan angle a (FIG. 3). After a beamcrosses angle a moving toward outer limit 24 a, the beam again is sensedby sensor A as it returns. The interval between these two crossings istime interval t0. Interval t0 is necessarily the period in which theamplitude of scan increases to its maximum and begins its return, asshown in FIG. 3. A second time interval t1 then occurs while the beammoves to be sensed by the second scanner, scanner B, known to be locatedwhere the beam is at scan angle b (FIG. 3). After crossing angle b thebeam again is sensed by sensor B as it returns. The interval betweenthese two crossings is time interval t2. The time internal t3 is thatbetween the second consecutive sensing of the beam by Sensor B and thenext sensing of the beam by Sensor A.

[0025] The amplitude, in terms of sweep angle in arbitrary units, is afunction of the ratio of time intervals t0 and t1 or t2 and t3. Thefunction defining amplitude is nearly linear when the values of all timeintervals t0, t1, t2, and t3 are nonzero. However, for the purposes ofthe control methods described, the amplitude and offset functions do notnecessarily have to be known explicitly.

[0026] The period is expressed as t0+t1+t2+t3, with the frequency ofoscillation being the reciprocal of the period. The difference betweent0 and t2 is a function of location of the sensors with respect to themedian of the beam sweep and defines the median offset.

[0027] The primary control method first determines the existing resonantfrequency. To do so the peak value of the alternating drive level to thetorsion oscillator is held constant, and the drive frequency is sweptover a small region around the expected nominal resonant frequency ofthe target device. This is done over a large enough range of frequenciesto cover the cumulative contributions to resonant frequency variation.These include device manufacturing and assembly tolerances, andtemperature. The control hardware is used to drive the scanner over thisfrequency range and at the same time, measure the resulting scanamplitude. Using a peak detection or inference, the controlleridentifies the frequency with the highest amplitude to be the resonantfrequency at the present operating conditions. This is most likely to bedone during printer power on reset (i.e., initialization at power on).With the resonant frequency identified and communicated to the printerengine, the electronic control sets the drive frequency.

[0028] In practice, the highest amplitude is found by finding either aminimum value of t1 or t3, or a maximum value of t0 or t2. Actualamplitude need not be calculated. Resonant frequency can be calculated,as defined as the reciprocal of period, and communicated to the printerengine. Alternatively, the resonant frequency can be obtained from thepart of the controller used to generate the drive frequencies based onthe resonant frequency requiring a lower drive level for a given scanamplitude.

[0029] It is not necessary to calculate scan characteristics in physicalterms. For instance, it is not required to calculate the actual scanamplitude in terms of degrees of mirror deflection or millimeters ofscan traverse. Instead the appropriate values of time intervals fromFIG. 3 are determined by the designer which will produce the desiredphysical scan length or angle needed for printing. Then, the controllermaintains these time intervals, in units of time counts, usingtraditional feedback control techniques.

Control Sequence

[0030]FIG. 5 illustrates the sequence of control in accordance with afirst control procedure of this invention. The first action is at poweron (Turn On), action 60. This then proceeds to action 62 in which theresonant frequency of the oscillator is determined. Then the resonantfrequency is monitored for change in decision 64, in the specificimplementation by searching for a change in amplitude.

[0031] If decision 64 is no, the sequence returns to decision 64 atregular intervals. If decision 64 is yes, action 66 is implemented,which is to restore amplitude and median offset, done by increasing ordecreasing drive level as required and adjusting the median of the drivelevel. The drive frequency is not changed. The sequence then proceeds toimplement decision 64 at regular intervals until decision 64 is againyes, at which point action 66 is implemented. This continuesindefinitely.

[0032]FIG. 6 illustrates the sequence of control in accordance with asecond control procedure this invention. As with the foregoing firstprocedure, the first action is at power on (Turn On), action 60. Thisthen proceeds to action 62 in which the resonant frequency of theoscillator is determined. This then proceeds to action 74 in which thedrive frequency is set offset close to the resonant frequency. Then theresonant frequency is monitored for change in decision 76, in thespecific implementation by searching for change in amplitude.

[0033] If decision 76 is no, the sequence returns to decision 76 atregular intervals. If decision 76 is yes, action 78 is implemented,which restores the close offset between resonant frequency and drivefrequency. The sequence then proceed to implement decision 76 at regularintervals until decision 76 is again yes, at which point action 78 isimplemented. This continues indefinitely.

Practical Advantages

[0034] The ideal resonant scanner controller would continuously detectshifts in the device's resonant frequency and adjust drive frequency tomatch the resonant frequency. If the controller can find the resonantfrequency of the scanner at every moment in time, it can always drivethe scanner with the minimum required power for the desired amplitude.However, as the amplitude profile of FIG. 4 suggests, two differentdrive frequencies, arranged symmetrically about the resonance peak, willproduce the same amplitude. This causes a problem with detectingresonant frequency shifts in real time. The disturbance to the systemthat the controller must track is the shift in resonant frequency,either above or below the resonance of the previous state.

[0035] Since the resulting amplitude can be produced by two differentfrequencies, the controller must decide which direction to direct thedrive frequency to move to the new resonant frequency. Withoutadditional information, the only way to detect the resonance peak is tosweep the drive frequency around the expected resonance and detect thepeak value of amplitude. However, the purpose of the controller is tomaintain expected scan characteristics continuously, especially duringprinting. Attempting to detect resonance through a frequency sweep whileprinting will undoubtedly create unacceptable print jitter.

[0036] One solution to this problem of this invention is the controlprocedure of FIG. 6, which always operates the scanner slightly offresonance. The scanner can be operated on the slopes on either side ofthe resonance peak, where a small region of the amplitude function isstrictly increasing or decreasing. Within this small region, there is aone to one mapping between a change in resonant frequency and a changein amplitude. As long as the electronic control can react fast enough toprevent a disturbance from pushing operation past the other side of theresonance peak, the control can maintain operation at a fixed distanceaway from the resonance frequency. The difficulty with this technique isthat desirable operation close to resonance carries a danger ofinstability, while a higher margin of safety requires operation fartherfrom resonance, where the required drive level increases rapidly tomaintain the desired scan amplitude.

[0037] Because of these difficulties, the solution of this invention inwhich the frequency is not changed is preferred except in theseinstances in which a particular system requires more power to maintainamplitude than is practicable and affordable.

[0038] Another proposed solution to the problem of detecting resonantfrequency in real time is to provide additional information to thecontroller. Since resonant frequency is a strictly increasing functionof temperature within the expected operating range of temperatures,adding temperature information to the controller inputs will solve theproblem of not knowing which direction to adjust drive frequency tomatch the drifting resonant frequency. However, this temperaturemeasurement must be of the scanner material itself, since thermalexpansion of the material is responsible for resonant frequency drift.Ambient temperature measurements in the enclosure surrounding the devicemay not adequately reflect transient temperature changes in the deviceand would therefore not be useful to the controls. In addition, it hasbeen shown in experiments that temperature changes on the order of 0.1degrees C. have a significant impact on scanner resonant frequencychanges. Unfortunately, sensing devices capable of this accuracy are notreadily available within the low cost constraints required by theprinting applications, so implementation of this technique is unlikely.

What is claimed is:
 1. A method of stabilizing a torsion oscillatorduring continuous, repetitive normal operation comprising, driving saidoscillator substantially at a first, resonant frequency of saidoscillator, said oscillator having a first amplitude, observing saidoscillator for change in the resonant frequency of said oscillator,continuing to drive said oscillator substantially at said firstfrequency while changing drive level to said oscillator to substantiallymaintain said first amplitude of said oscillator, and continuing saidobserving and said changing drive level to substantially maintain saidfirst amplitude during said continuous repetitive normal operation ofsaid oscillator.
 2. The method as in claim 1 in which said observing isby determining the amplitude of oscillations of said torsion oscillator.3. The method as in claim 1 in which said torsion oscillator supports amirror and said determining is by observing light reflected from saidmirror as said mirror is rotated by said torsion oscillator.
 4. Themethod as in claim 2 in which said torsion oscillator supports a mirrorand said determining is by observing light reflected from said mirror assaid mirror is rotated by said torsion oscillator.
 5. The method as inclaim 1 in which said continuing to drive said oscillator includes thestep of adjusting the median offset of said oscillator by changing themedian of said drive level.
 6. The method as in claim 2 in which saidcontinuing to drive said oscillator includes the step of adjusting themedian offset of said oscillator by changing the median of said drivelevel.
 7. The method as in claim 3 in which said continuing to drivesaid oscillator includes the step of adjusting the median offset of saidoscillator by changing the median of said drive level.
 8. The method asin claim 4 in which said continuing to drive said oscillator includesthe step of adjusting the median offset of said oscillator by changingthe median of said drive level.
 9. A method of stabilizing a torsionoscillator during continuous, repetitive normal operation comprising,driving said oscillator at an offset frequency close to the resonantfrequency of said oscillator, observing said oscillator for change inthe resonant frequency of said oscillator, changing said offsetfrequency to be offset close to said changed resonant frequency, andcontinuing said observing and said changing said offset during saidcontinuous repetitive normal operation of said oscillator.
 10. Themethod as in claim 9 in which said observing is by determining theamplitude of oscillations of said torsion oscillator.
 11. The method asin claim 9 in which said torsion oscillator supports a mirror and saiddetermining is by observing light reflected from said mirror as saidmirror is rotated by said torsion oscillator.
 12. The method as in claim10 in which said torsion oscillator supports a mirror and saiddetermining of amplitude is by observing light reflected from saidmirror at least two, separated sensors as said mirror is rotated by saidtorsion oscillator.